Macrophages are found in advanced atherosclerotic plaques, and have been shown to have an increased abundance of free cholesterol (FC) accumulation. FC is a potent inducer of macrophage apoptosis. Aim I will elucidate the role of the MKK3/p38 MAPK pathway in induction of the Unfolded Protein Response (UPR), and distal apoptotic pathways in FC-loaded macrophages. Peritoneal macrophages from wild-type mice, or mice deficient in MKK3 and p38alpha, will be used to ask if this signaling pathway is upstream of cholesterol trafficking to the endoplasmic reticulum (ER), or depletion of ER Ca2+ stores in response to FC loading. Aim II will determine the role of the MKK3/p38 MAPK pathway in atherosclerotic lesion progression using an in-vivo model of atherogenesis. Mkk3-/- and p38alpha-floxed/LysMCre mice crossed onto an Apoe-/- background will be used to determine if blocking the MKK3/p38 MAPK pathway results in inhibition of macrophage apoptosis in-vivo, and alters atherosclerotic lesion morphology. The main objective of this proposal is to elucidate the molecular mechanisms connecting the MKK3/p38 MAPK pathway, induction of the UPR, and determine if this pathway influences macrophage apoptosis and atherosclerosis in-vivo.